The invention relates to a modular joint prosthesis having a head part and at least one shaft part which are insertably connectable with one another by means of a cone connection.
Such joint prostheses are produced in the most various shapes and sizes, to enable the most precise possible adaptation to the shape of the bone of a particular patient, as well as the current status of the bony tissue, taking the clinical picture into account.
By means of a multi-part embodiment with nonpositive connection of the corresponding individual parts in the proximal region, the adaptation can be accomplished optimally. At the same time, positioning of the joint head is possible regardless of the shaft diameter.
From European Patent Disclosure EP-B1 0 243 298, a kit for a shaft prosthesis is known, which has a head part that can be provided with a joint ball, an end part anchored in the bone, and an intermediate part that can be positioned between the two. All the parts have conical bores or complementary pegs, and as a result the prosthesis can be assembled by making conical insert connections. The head part and intermediate part each have an axial through bore.
The end part is provided with releasable engagement means, for the sake of transmitting a force in the axial direction. When the individual parts are put together, the corresponding bores are aligned axially in the direction of the shaft. The individual parts of the prosthesis are put together using a tie rod that transmits a force in the axial direction and that penetrates both the head part and succeeding shaft parts and can be screwed into the threaded bore of the end part. As a result, the head part, or the intermediate part, and the end part are firmly tightened against one another, so that loosening of the individual parts of the prosthesis from the mechanical strain during use need not be feared.
The above-described embodiment has the disadvantage that in such a prosthesis, the insert connection between the individual parts cannot be undone or released in the implanted state, even though this is sometimes necessary for medical reasons--for instance for resetting or replacing the head part. The conical insert connections, even after prior removal of the tie rod provided in the interior of the shaft, are held together so firmly that, if disassembly becomes necessary, damage to the femur or at least undesired loosening of the shaft is sometimes unavoidable, which is detrimental to the patient.